I really don't want to offend ace holmes, as I believe he is very skilled in engineering and his April 26th engineering details will be very educational for many of us. Even if his conclusions are unworkable, someone may find a way to advance human technology from the spring board ace holmes is offering. This is called brain storming. In brainstorming stupid objections are permitted, so I will proceed with what may be my really stupid comments. The upper temperature of a radio isotope is determined mostly by the ambient temperature, R value of the insulation and the temperature at which the insulation melts or sublinates. The watts, BTUs or other new fangled units of heat is determined by the mass of the pellet and which isotope. Likely about one kilowatt per pound for PU238, which is why I suggested 90 pounds of Pu 238 might put 50 kilowatts to the wheels of a typical SUV, using a turbine to convert heat to rotory motion. A few other isotopes likely produce more heat than one kilowatt per pound, but most also produce neutrons and/or gamma rays which are difficult to shield. Apparently Pu238 and strontium 90 produces little or no hard to shield radiation.
Diamond burns much like charcoal in an oxygen enviornment above about 500 degrees c = 932 f, so a diamond shield would likely fail in a bad car fire. A bad car fire is unlikely in a steel car without a flamable fuel, so that may be low risk. Diamond is however brittle and I think it can shatter from a sudden impact with softer materials. Perhaps the casing used for flight recorders is adequate protection. Strontium boils at 1384 c, but has three allotropic forms at 235c and 540c. That likely is irrelevent as metalic strontium reacts with water and oxygen less vigorously than sodium, but more vigorously than calcium. Strontium floride boils at 2489c which is rare even in hot fires, so the strontuim floride crystal is likely to remain intact in all but the worst scenarios, except it is slightly soluable in water 0.11 grams per liter in either hot or cold water. Neil